A frequent way today to restore a damaged limb, such as lost tooth, is to install a fixture in the adjacent bone tissue and replace the damaged parts. In this respect, for a successful result, the fixture should become fully stable and correctly joined to the bone. The term osseointegration is used for this joining effect, the basic meaning of this term being the bone tissue growth into the fixture surface. The two major contributors to this joint are a mechanical joint and an organic joint. The former being generally influenced by the macro geometry of the bore into which the fixture is installed, and by the macro geometry of the fixture, and is a direct effect of how well these two work together. The latter one being a continuously evolving and developing effect, particularly the time immediately after installation, and being generally influenced by how well the micro surface structure of the fixture interacts with the bone tissue.
Due to ingrowth there will be an interlocking effect between the bone and the fixture. Also, the mechanical joint is developed over time since the bone tissue, under ideal conditions, may grow into surface cavities of the fixture, and grow into voids left between the fixture and the bore after installation.
Interaction of mechanical and organic aspects will affect the bone ingrowth and joint between the bone and the projecting surface structure of the fixture. The projecting surface structure may be in the form of threads, annular ridges, lines or patterns of beads etc. Further a blasted, or otherwise roughened, surface of the fixture will provide advantageous conditions for this process.
Dental fixtures are subjected to functional forces. These forces are transmitted to the surrounding bone as compressive forces, tensile forces and shear forces at the fixture/bone interface. The higher the ability of the interface to withstand these three types of forces the bigger the loads the fixture can support. The ability of the interface to resist compressive forces is relatively large. In comparison, the ability of the interface to stand tensile forces is insignificant or low. As regards the third force, research has been spent on trying to enhance the fixture/bone interface shear strength through modification of the micrometer (μm)-sized roughness of the fixture surface.
The roughness of the fixture surface may be regarded as comprising pits and protrusions. Bone is allowed to grow into the pits on the fixture surface and create retention. The bone which has grown into the pits may be referred to as bone plugs. Correspondingly, protrusions will engage the bone and create retention.
It has been found that despite the effort of providing increased retention by means of micro-roughening the fixture surface, the resulting bone plugs and the bone portions engaged by fixture protrusions are often broken in use, whereby the intended functionality of the roughness is at least partly lost.
An object of the invention is therefore to provide a fixture which enables formation of bone that create long term retention, particularly in cortical bone tissue. These and other objects, which will become apparent in the following, are accomplished by the invention as defined in the accompanying claims.